Participants in this research study include children diagnosed with Smith-Lemli-Opitz syndrome and children without this disorder who will serve as control subjects and provide normative data. The Smith-Lemli-Opitz Syndrome results in a very low blood cholesterol level and a high blood level of 7-dehydrocholesterol (another form of cholesterol). The purpose of the study is to feed participants a diet that contains no cholesterol and a diet that is high in cholesterol and determine the effects on the child's blood cholesterol level, the blood level of 7-dehydrocholesterol and the absorption and metabolism of cholesterol. The long term goal is to determine if the dietary cholesterol reduces any of the usual symptoms of the Smith-Lemli-Opitz Syndrome. In addition, in patients over 2 years of age, genetic studies (DNA) will be done to help identify the gene for Smith-Lemli-Opitz syndrome and to study mutations (changes) in the gene.